1. Field of the Invention
The present invention relates to an ink-jet recording apparatus provided with a recording head having nozzles capable of jetting ink particles through the nozzle.
2. Description of the Related Art
An ink-jet recording apparatus generates relatively low noise during a printing operation and is capable of forming small dots with a high density. Accordingly, the ink-jet recording apparatus has been used prevalently in recent years for printing images, including full-color images.
The ink-jet recording apparatus comprises an ink-jet recording head supplied with ink from an ink cartridge, and a sheet feed mechanism for moving a recording sheet relative to the recording head. A carriage mounted with the ink-jet recording head is moved in a direction along the width of the recording sheet and ink particles are jetted onto the recording sheet by the ink-jet recording head for recording (printing). A full-color ink-jet recording apparatus is provided with black, yellow, cyan and magenta ink-jet recording heads mounted on a carriage and capable of jetting black, yellow, cyan and magenta ink particles, respectively. The full-color ink-jet recording apparatus is capable of full-color printing by jetting those color inks at appropriate ratios, as well as text printing by forming black letters.
The ink-jet recording head jets ink particles by pressure produced in a pressure chamber through nozzles onto a recording sheet for printing. Therefore, it is possible that operation of the ink-jet recording head results in faulty printing due to the increase of the viscosity of the ink or the solidification of the ink, resulting from the evaporation of the solvent of the ink through the nozzles. Faulty printing will also be caused by the adhesion of dust to the nozzles or the formation of bubbles in the ink.
The ink-jet recording apparatus is provided with a capping means for sealing up the openings of the nozzles of the recording head when the recording head is not in printing operation, and a cleaning device for cleaning a nozzle plate when necessary.
The capping means functions as a cover for preventing the ink from drying in the nozzles while the ink-jet recording apparatus is not in printing operation. The capping means further functions to remove the ink solidified in the nozzles which clogs the nozzles and to remove bubbles formed in ink passages and causes a faulty ink jetting operation, in cooperation with a suction pump, by bringing the capping means in close contact with the nozzle plate and by applying a negative pressure to the nozzles to suck out the ink clogging the nozzles, when the nozzles are clogged.
The suction cleaning operation for forcibly sucking out the ink from the clogged nozzles of the recording head and for removing bubbles from the ink passages is generally called a cleaning operation. The cleaning operation is carried out before resuming the printing operation after a long interruption of the printing operation. The cleaning operation is also carried out when an operator operates a cleaning switch to clean the nozzles when the print quality of printed images deteriorates. A wiping operation is carried out to wipe the surface of the recording head with a wiping member consisting of elastic plates such as rubber plates, after removing the ink from the nozzles by the cleaning (suction) operation.
The recording head can forcibly jet ink particles when a driving signal unrelated with printing operation is applied. This jetting operation is generally called a flushing operation. The flushing operation is performed to regulate menisci of the ink at the outlet openings of the nozzles of the recording head when the menisci are disturbed by the wiping operation after the cleaning operation. The flushing operation is also performed to discharge the mixed ink forced to flow in reverse into the nozzles by the wiping operation, from the nozzles. The flushing operation is performed periodically to prevent the nozzles through which only a small amount of the ink is jetted during the printing operation from being clogged with the ink due to an increase in the viscosity of the ink.
The ink-jet recording apparatus is provided with, for example, a recording head as shown in 5c FIG. 10. FIG. 10 is a sectional view showing one of the ink passages of a recording head 5. As shown in FIG. 10, a practical multinozzle recording head 5 has ink jetting nozzles arranged in rows, each of the rows is formed by combining an ink passage and a nozzle.
A lower electrode 5b is formed on a surface of a vibrating plate 5a. A piezoelectric member 5c, such as a PZT, is placed on the surface of the lower electrode 5b. An upper electrode 5d is formed on a surface of the piezoelectric member 5c. The piezoelectric member 5c expands or contracts by a driving signal applied thereto through the lower electrode 5b and the upper electrode 5d, and then the vibrating plate 5a is driven (distorted) for vertical movement in FIG. 10.
A spacer 5e underlies the vibrating plate 5g. The spacer Se is provided with a recess in its surface facing the vibrating plate 5a to form a cavity (pressure chamber) 5f under the vibrating plate 5a. 
An ink supply port forming plate 5g underlies the spacer 5e. The plate 5g is provided with an ink supply port 5h opening into the cavity 5f. 
A spacer 5i underlies the ink supply port forming plate 5g. The spacer 5i is provided with a hollow for forming a reservoir (common ink chamber) 5j. 
A nozzle plate 5m provided with a nozzle 5k underlies the spacer 5i. The ink supply port forming plate 5g and the spacer 5i are provided with openings forming a straight ink passage 5n extending between the cavity 5f and the nozzle 5k. The spacer 5e, the ink supply port forming plate 5g and the spacer 5i are bonded together with adhesive layers.
As mentioned above, the vibrating plate 5a vibrates vertically, as shown in FIG. 10, by the expansion and contraction of the piezoelectric member 5c. When electric power is supplied to the piezoelectric member 5c, the vibrating plate 5a moves vertically downward. Consequently, pressure is applied to the ink contained in the cavity 5f to force the ink to flow through the ink passage 5n and the ink is jetted through the nozzle 5k as ink particles. When electric charges are discharged from the piezoelectric member 5c, the vibrating plate Sa returns to an original state thereof Consequently, the cavity 5f expands, the ink is supplied from the ink reservoir 5j (the common ink chamber) through the ink supply port 5h into the cavity 5f to replenish the cavity 5f with the ink for the next printing cycle.
Thus, the volume of the cavity 5f is changed by the piezoelectric member 5c to replenish the cavity 5f with the ink supplied from the ink reservoir 5j and to jet the ink supplied from the cavity 5f through the ink passage 5n through the nozzle. 5k as ink particles.
FIGS. 11(a) and 11(b) are sectional views of the recording head for explaining the behavior of ink particles jetted in the flushing operation.
As shown in FIG. 11(a), a main ink particle M and an ink string following the main ink particle M are spewed out from the nozzle 5k when the volume oft he cavity 5f is reduced. A part of the ink string changes into a plurality of small ink particles S because of the surface tension of the ink as shown in FIG. 11(b). Those small ink particles S are referred to also as satellite particles.
Generally, the small ink particles S fly at low speed, have very small weight and are liable to float in air as ink mist. The ink mist may contaminate the interior of the recording apparatus, and may be discharged outside through an opening of the recording apparatus, such as an exhaust opening for a cooling fan, to contaminate the peripheral equipment.
When the recording apparatus has a second flushing region on the opposite side of the capping means with respect to a printing region, there is a limit to the amount of the flushing ink discharged into the capping means, and a large amount of flushing ink must be discharged into the second flushing region.
Especially, when an opening 13 is formed in a sheet guide member 8 disposed opposite to the nozzles 5k of the recording head 5, and an ink absorbing member 14 is disposed on the side of its bottom in the second flushing region as shown in FIG. 12, the distance between the surface in which the nozzles 5k open and the ink absorbing member 14 for absorbing the flushed ink is as long as several tens of millimeters.
When the ink absorbing member 14 is relatively distant from the surface in which the nozzles 5k open, the small ink particles S may drift away before they reach the ink absorbing member 14. Thus, the small drifting ink particles S may contaminate the components. Particularly, the foregoing problem is conspicuous in the recent recording apparatuses which use ink particles of the least possible amount of ink to print images with a high print quality.
Ink particles jetted through the nozzles are charged to not a small extent and it is possible that ink particles are accelerated by static electricity generated by a driving unit included in the recording apparatus.
It is also possible that the jetted ink particles are accelerated by air currents generated by an exhaust fan included in the recording apparatus, which fan is disposed to suppress the temperature rise of the recording apparatus.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide an ink-jet recording apparatus capable of effectively suppressing the formation of minute ink particles that may float in mist when carrying out a flushing operation and of preventing the contamination of the ink-jet recording apparatus itself and the peripheral equipment.
According to a first aspect of the present invention, an ink jet recording apparatus comprises: a flushing signal generating unit that generates a flushing signal, and a recording head provided with a nozzle and capable of jetting ink particles through the nozzle on the basis of the flushing signal, wherein the flushing signal causes the recording head to jet ink particles through the nozzle so that each of the ink particles is a main ink particle.
Since the flushing signal makes the recording head jet only main (large) ink particles through the nozzles and does not make the recording head jet minute ink particles, contamination caused by the minute ink particles can be avoided.
Generally, the flushing signal is a periodic signal for efficient flushing. The periodic signal may have periodic pulses and each of the pulses may have a trapezoidal waveform having a first inclined section, a potential maintaining section continuous with the first inclined section and a second inclined section continuous with the potential maintaining section. In this case, the duration of the trapezoidal pulse, the inclination of the first inclined section, the potential level of the potential maintaining section and the inclination of the second inclined section, as well as the frequency of the flushing signal, may be used as controllable parameters.
More concretely, it is preferable that the first inclined section is inclined gently and the second inclined section is inclined sharply. In this case, the allowable ranges for the frequency of the flushing signal, the duration of the trapezoidal pulse, and the level of the potential maintaining section may be relatively wide.
According to a second aspect of the present invention, an ink-jet recording apparatus comprises: a flushing signal generating unit that generates a flushing signal, and a recording head provided with a nozzle and capable of jetting ink particles through the nozzle on the basis of the flushing signal, wherein the flushing signal causes the recording head to jet ink particles through the nozzle so that each of the ink particles has a momentum greater than a predetermined value.
Since the flushing signal makes each of the recording head jet ink particles have a momentum greater than the predetermined value through the nozzles i.e. even a minute ink particle has a momentum greater than the predetermined value, contamination caused by conventional minute ink particles can be avoided.
Generally, the flashing signal is also a periodic signal for efficient flashing. The periodic signal may have periodic pulses and each of the pulses may have a trapezoidal waveform having a first inclined section, a potential maintaining section continuous with the first inclined section and a second inclined section continuous with the potential maintaining section. In this case, the duration of the trapezoidal pulse, the inclination of the first inclined section, the potential level of the potential maintaining section and the inclination of the second inclined section, as well as the frequency of the flushing signal, may be used as controllable parameters.
More concretely, it is preferable that the first inclined section is inclined gently and the second inclined section is inclined sharply, because the allowable ranges for the frequency of the flushing signal, the pulse width of the trapezoidal pulse and the level of the potential maintaining section are wide when the first inclined section is inclined gently and the second inclined section is inclined sharply.
According to a third aspect of the present invention, an ink-jet recording apparatus comprises: a flushing signal generating unit that generates a flushing signal, and a recording head provided with a nozzle and capable of jetting ink particles through the nozzle on the basis of the flushing signal, wherein the flushing signal causes the recording head to intermittently jet the ink particles through the nozzle so that the ink particles include sets of a main ink particle and minute ink particles after the main ink particle, and minute particles of a set combine with a main ink particle of a following set in a range of a predetermined distance from the nozzle.
Since the flushing signal makes the recording head jet ink particles through the nozzle so that the minute ink particle merge with the main ink particle following the minute ink particle, contamination caused by conventional minute ink particles can be avoided.
Generally, the flushing signal is also a periodic signal for efficient flushing. The periodic signal may have periodic pulses and each of the pulses may have a trapezoidal waveform having a first inclined section, a potential maintaining section continuous with the first inclined section and a second inclined section continuous with the potential maintaining section. In this case, the duration of the trapezoidal pulse, the inclination of the first inclined section, the potential level of the potential maintaining section and the inclination of the second inclined section, as well as the frequency of the flushing signal, may be used as controllable parameters.
More concretely, it is preferable to increase the frequency of the flushing signal to about 10 kHz. In this case, the allowable ranges for the frequency of the flushing signal, the duration of the trapezoidal pulse and the level of the potential maintaining section may be relatively wide.
Preferably, the ink-jet recording apparatus may comprise a capping means for sealing the nozzle of the recording head, and the ink particles jetted by the recording head through the nozzle when driven by the flushing signal are caught by the capping means.
Alternatively, the ink-jet recording apparatus may comprise a member provided with an opening opposite to which the nozzles of the recording head can be disposed; and an ink absorbing member disposed on the side of a bottom part of the opening, wherein the ink particles jetted by the recording head through the nozzle on the basis of the flushing signal fly through the opening and are caught by the ink absorbing member.
When the recording head is provided with a plurality of nozzles, respectively, for a plurality of inks, it is preferable that different flushing signals are used for the nozzles for jetting the different inks, respectively.
When the recording head is provided with a plurality of nozzles, respectively, for a plurality of inks and the recording apparatus has a plurality of flushing regions, it is preferable that ink particles of the different inks jetted by the recording head through the plurality of nozzles are caught in the different flushing regions, respectively.
The different inks mean inks having different colors, or different viscosities, or different surface tensions, or the like.
Preferably, the ink-jet recording apparatus is provided with a fan for preventing a temperature rise of the recording apparatus; and a fan control means for stopping the fan during a flushing operation in which the recording head jets ink particles through the nozzle. In this case, it is preferable that the fan control means keeps the fan stopped at least until the ink particles jetted by the recording head through the nozzle on the basis of the flushing signal arrive at and are caught by the ink absorbing member.